This invention relates to land-based gas turbine engines and specifically, to a “can-annular” combustor arranged with one center fuel nozzle surrounded by several radially outer fuel nozzles. More specifically, the invention relates to configurations of the center nozzle and outer nozzles so as to avoid flame attachment for selected nozzles at all operating conditions by incorporating a swirler device with a deliberately low-swirl aerodynamic design.
In gas turbine combustors utilizing DLN (dry low NOx) technology, it has been observed that there is a strong linkage between combustor dynamics (unsteady pressure fluctuations) and the “attachment” or “detachment” of the flame from one or several nozzles. An attached flame is anchored closely to the nozzle exit by the recirculation pattern in the vortex breakdown region. A detached flame is not anchored and exists several inches downstream of the nozzle exit. Attachment or detachment can be influenced by the fuel-air ratio, i.e., richer nozzles tend to run attached while leaner nozzles tend to run detached. In some designs, at the normal operating condition, it is not possible to provide sufficient fuel to all nozzles to keep all flames attached. In the process of tuning fuel splits, i.e., adjusting the relative quantity of fuel supplied to each nozzle, it has been found that optimum dynamics are obtained with some nozzle flames detached and some attached, but that sometimes large increases in dynamics are encountered where one or more nozzles are near their transition between flame attachment and flame detachment.
In accordance with current practice, all of the nozzles in a combustor assembly incorporate swirlers that have vanes shaped to provide a nominally high-swirl angle in order to maximize the aerodynamic stability via vortex breakdown. Specifically, it is common practice for the vane swirl angle to be in the range of 40°–50° relative to the longitudinal axis of the nozzle. In general, high-swirl angles promote a wider range of conditions at which the flame will attach. At the same time, fuel splits are used to tune in the field or in the lab to find the combination of attached and detached flames that results in the best dynamics—NOx tradeoff.